(1) Definition. A lack of sufficient oxygen is deemed to exist if the atmosphere at sea level has less than 19.5% oxygen by volume or has a partial pressure of oxygen of 148 millimeters of mercury (mm Hg) or less. This may deviate when working at higher elevations and should be determined for an individual location. Factors such as acclimatization, physical conditions of the persons involved, etc., must be considered for such circumstances and conditions.

(2) Entering areas with possible oxygen deficient atmospheres. Workers entering any area where a lack of sufficient oxygen is probable must be supplied with and must use approved equipment (for specific requirements see applicable provisions of chapters 296-62, 296-307 (Part-U3), 296-809 and 296-841 WAC) capable of providing safe respirable air, or prior to entry and at all times when workers are in such areas a sufficient supply of safe, respirable air must be provided. All workers so exposed must be under constant observation. If the oxygen content is unknown or may change during occupation, tests must be required prior to and during occupation of questionable areas.